SCAN: A Dynamic, Scalable, and Efficient Content Distribution Network

Yan Chen, Randy H. Katz,John D. Kubiatowicz

{yanchen, randy, kubitron}@CS.Berkeley.EDU

EECS DepartmentUC Berkeley

Outlines

• Motivation

• Goal and Challenges

• Previous Work

• SCAN Architecture and Components

• Evaluation Methodology

• Results

• Conclusions

Motivation Scenario: World Cup 2002

Goal and Challenges

• Dynamic choice of number and location of replicas – Clients’ QoS constraints: latency, staleness– Servers’ capacity constraints

• Efficient resource consumption– Small delay, bandwidth consumption for replica update– Small replica management cost

• Scalability: millions of objects, clients and servers

• No global network topology knowledge

Provide content distribution to clients with good latency and staleness, while retaining efficient and balanced resourceconsumption of the underlying infrastructure

Previous Work• Replica Placement

– Research communities: optimal static replica placement• Assume clients’ distributions, access patterns & IP topology• No consideration for clients’ QoS or servers’ capacity

constraints

– CDN operators: un-cooperative, ad hoc placement • Centralized CDN name server cannot record replica locations

– place many more than necessary (ICNP ’02)

• Update Multicast– No inter-domain IP multicast– Most application-level multicast (ALM) unscalable

• Split root as common solution, suffers consistency overhead

adaptivecoherence

data plane

network plane

datasource

Web contentserverCDN server

client

replica

always updatecache

SCAN: Scalable Content Access Network

DOLR mesh

Components of SCAN• Decentralized Object Location & Routing (DOLR)

– Properties needed• Scalable location with guaranteed success

• Search with locality

– Improve the scalability of d-tree: each member only maintains states for its parent and direct children

• Simultaneous Dynamic Replica Placement and d-tree Construction– Replica search: Singular, Localized or Exhaustive– Replica placement on DOLR path: Lazy or Eager

parent candidate

data plane

network plane

c

s

DOLR path

Replica Search

proxy

DOLR mesh

• Singular Search

Replica Search

parent candidates

• Localized search

data plane

network plane

c

s parent

siblingserver child

proxy

DOLR path

client child

• Greedy load distribution

DOLR mesh

data plane

network plane

c

sproxy

DOLR path first placement choice

Replica Placement: Eager

DOLR mesh

Replica Placement: Lazy

data plane

network plane

c

sproxy

DOLR path

client child

first placement choice

DOLR mesh

Evaluation of Alternatives• Two dynamic overlay approaches

– Overlay_naïve: Singular search + Eager placement– Overlay_smart: Localized search + Lazy placement

• Compared with static placement + IP multicast– Overlay_static: With global overlay topology– IP_static: With global IP topology (ideal)

• Metrics– Number of replicas deployed, load distribution– Multicast performance: Relative Delay Penalty (RDP)

and bandwidth consumption– Tree construction traffic (packets and bandwidth)

Methodology• Network Topology

– 5000-node network with GT-ITM transit-stub model– SCAN nodes placed randomly or on transit nodes

• NS-like Packet-level Network Simulations • Workloads

– Synthetic flash crowd: all clients access a hot object in random order

– Real Web server traces: NASA and MSNBC

Web Site Period Duration # Requests # Clients # objects

MSNBC 8/2/1999 10–11am 1.6M 140K 4186

NASA 7/1/1995 All day 64K 5177 3258

Methodology: Sensitivity Analysis• Various Client/Server Ratio• Various Server Density• Various Latency & Capacity Constraints• Various Network Topologies

– Average over 5 topologies with different setup

• All Have Similar Trend of Results– Overlay_smart has close-to-optimal (IP_static)

number of replicas, load distribution, multicast performance with reasonable amount of tree construction traffic

Number of Replicas Deployed and Load Distribution

• Overlay_smart uses only 30-60% of replicas than overlay_naïve and very close to IP_static• Overlay_smart has two times better load distribution than od_naïve, overlay_static and very close to IP_static

Multicast Performance

• 85% of overlay_smart Relative Delay Penalty (RDP) less than 4

• Bandwidth consumed by overlay_smart is very close to IP_static, and is only 1/3 of bandwidth by overlay_naive

Tree Construction TrafficIncluding “join” requests, “ping” messages, replica

placement and parent/child registration

• Overlay_smart consumes 3 - 4 times of traffic than overlay_naïve, and the traffic of overlay_naïve is quite close to IP_static• Far less frequent event than access & update dissemination

Conclusions• P2P networks can be used to construct CDNs• SCAN: Scalable Content Access Network with

good QoS, efficiency and load balancing– Simultaneous dynamic replica placement & d-tree

construction – Leverage DOLR to improve scalability and locality

• In particular, overlay_smart recommended– Localized search + Lazy placement– Close to optimal number of replicas, good load

distribution, low multicast delay and bandwidth penalty at the price of reasonable construction traffic

Results on Web Server Traces

• Limited simulations, most URLs have very few requests • Overlay_smart uses only one third to half replicas than overlay_naïve for hot objects

data plane

network plane

datasource

Web contentserver

CDN server

client

replica

always update

cache

SCAN: Scalable Content Access Network

adaptivecoherence

DOLR mesh

parent candidate

data plane

network plane

c

s

DOLR path

Replica Search

proxy

DOLR mesh

• Singular Search

Replica Search

parent candidates

• Localized search

data plane

network plane

c

s parent

siblingserver child

proxy

DOLR path

client child

• Greedy load distribution

data plane

network plane

c

sproxy

Tapestry overlay path first placement choice

parent candidate

Dynamic Replica Placement: naïve

Tapestry mesh

• Singular Search • Eager Placement

Dynamic Replica Placement: smart• Localized search • Lazy placement

• Greedy load distribution

data planeparent candidates

network plane

c

s parent

siblingserver child

proxy

Tapestry overlay path

client child

first placement choice